// of pods into each bucket.
  // We define a domain as a particular instance of a topology.
  // Also, we define an eligible domain as a domain whose nodes meet the requirements of
  // nodeAffinityPolicy and nodeTaintsPolicy.
  // e.g. If TopologyKey is "kubernetes.io/hostname", each Node is a domain of that topology.
  // And, if TopologyKey is "topology.kubernetes.io/zone", each zone is a domain of that topology.
  // It's a required field.